Single mode fibre and method of manufacture

ABSTRACT

Single mode optical fibre for operation in the wavelength range 1.5 to 1.7 microns is made by first depositing an optically absorbing layer of silica doped with oxides of boron and/or phosphorus upon the bore of a silica substrate tube. The optical absorbing layer has a higher refractive index than silica, and on it is deposited a transparent optical cladding layer of matched index, and then a higher index optical core layer. The bore of the coated tube is then collapsed to form a solid cross-section optical fibre preform from which optical fibre can be drawn.

FIELD OF THE INVENTION

This invention relates to the manufacture of optical fibre preforms andoptical fibres. It is concerned particularly with the manufacture ofsingle mode optical fibres capable of operation in the free-spacewavelength range 1.5 to 1.7 microns, that have optical cores and opticalcladdings of vapour deposited doped silica.

One of the particular advantages of using silica as a material fromwhich to manufacture optical fibres is that it can be made by vapourreaction process in a manner that permits close control of the amount ofany other materials incorporated into the material with silica. This isparticularly important having regard to the fact that certain impuritiesat concentrations of less than 1 part per million can still have markedeffects upon fibre transmission loss. In one of the preferred methods ofmanufacture, materials to form the cladding glass and the core glass aredeposited by vapour reaction upon the bore of a silica substrate tubewhich subsequently has its bore collapsed and is drawn into fibre.Methods of producing single mode fibre have been described for instanceby T. Miya et al. in Electronics Letters Feb. 15, 1979, Volume 15, No.4, pp. 106-8 and by B. J. Ainslie et al. in Electronics Letters July 5,1979, Volume 15, No. 14, pp. 411-3. In both instances the material usedto form the optical cladding had a composition chosen to have arefractive index matching that of the silica substrate tube onto whichthe cladding glass was deposited. This matching of index avoids theformation of a structure that would support cladding modes (modes guidedby the interface between the cladding and the substrate) in addition tothe desired single core mode. When using the process of vapour reactionto deposit core and cladding glasses on the bore of a substrate tube, itis generally desirable to use undoped silica as the material from whichto construct the tube because its relatively high melting point easesthe problem of trying to prevent unacceptable distortion of the tubeoccurring when high temperatures are involved in the deposition process.Under these circumstances any need to match the index of the opticalcladding material with that of the substrate tube has the result ofimposing a specific value for the refractive index of the cladding.

SUMMARY OF THE INVENTION

The present invention is concerned with structures which permit the useof an optical cladding whose index does not match that of the substratetube, and thereby provide a greater flexibility in design havingparticular regard to numerical aperture and the possibility of balancingmaterial dispersion against waveguide dispersion to give zero totaldispersion.

According to the present invention there is provided a method of makingan optical fibre preform wherein an optical absorption glass layer ofdoped silica that includes oxides of boron and/or phosphorus and has anindex greater than that of silica is deposited by vapour reaction uponthe bore of a silica substrate tube, wherein a transparent opticalcladding glass layer of doped silica that includes germania and has arefractive index less than, equal to, or not substantially greater thanthat of the absorption layer is deposited by vapour reaction upon theabsorption layer, wherein a transparent optical core glass layer ofdoped silica that includes germania and has a refractive index greaterthan that of the cladding layer is deposited by vapour reaction upon thecladding layer, wherein each of said vapour reactions is a reaction fromwhich hydrogen and its compounds are excluded, and wherein the bore ofthe thus coated tube is collapsed to form a solid cross-section opticalfibre preform, and the relative amounts and compositions of the core andcladding glasses deposited are such that single mode optical fibre iscapable of being drawn from the preform that is capable of single modeoperation at selected wavelengths in the free-space wavelength range of1.5 to 1.7 microns in which substantially all of the optical powerassociated with the evanescent field of said single mode propagates insaid cladding glass.

BRIEF DESCRIPTION OF THE DRAWINGS

There follows a description of the manufacture of a single mode fibreemobodying the invention in a preferred form. This description isprefaced with an explanation of the background to the invention. In thedescription and explanation reference is made to the accompanyingdrawings, in which:

FIG. 1 is the spectral characteristic of a multimode graded index fibre,

FIG. 2 is the spectral characteristic of a single mode fibre, and

FIG. 3 depicts a schematic cross-section of an internally coatedsubstrate tube prior to collapse of its bore to form a preform fromwhich optical fibre can be drawn.

DETAILED DESCRIPTION OF THE INVENTION

In order to prevent the guiding of cladding modes the refractive indexof the cladding layer is made equal to or less than that of theabsorbing layer that surrounds it. Cladding modes will be guided if therefractive index of the cladding layer is greater than that of theabsorbing layer, but, provided the index is not substantially greater, asignificant proportion of the optical power of any cladding modes willpropagate in the evanescent field present in the absorbing layer. Underthese conditions the attenuation of cladding modes is so high that thepresence of a cladding mode waveguiding structure can be tolerated. Theinterface between the absorption layer and the substrate similarlyprovides a waveguiding structure, but, in this instance also, theattenuation of the absorption layer prevents any effective propagationof unwanted modes.

One of the important factors affecting the optical transmission offibres produced by vapour deposition upon the bore of a tube is thelevel of contamination by hydroxyl groups. If water is one of thereaction products of the vapour reaction used for deposition such groupsmay become directly incorporated into the deposited material. Thissource of contamination is in the present instance avoided by choosingvapour reactions from which hydrogen and its compounds are excluded. Oneclass of suitable reactions involves the direct oxidation of halides oroxyhalides with oxygen. For this purpose the reagents may be entrainedin oxygen and caused to flow down the substrate tube. The reaction doesnot proceed at room temperature but may be promoted in the localisedregion of a short high temperature zone provided, for instance, by anoxy-hydrogen flame. This zone is slowly traversed a number of timesalong the tube so as to build up a uniform thickness of clear glassydeposit along its length.

The material of the substrate tube is liable to be quite heavilycontaminated with hydroxyl groups, and therefore the absorption layerhas the additional function of serving as a diffusion barrier to limitthe diffusion of hydroxyl groups from the substrate tube into thecladding and core glasses. For this purpose it is desirable to choose aglass composition for the absorption layer that can be deposited at arelatively low temperature and high deposition rate. It is found thatthe direct oxidation of silicon tetrachloride with oxygen proceeds at arelatively slow rate, and a relatively high deposition temperature isrequired to produce a clear fused deposit. However by co-depositing thesilica with one or more oxides of boron, phosphorus, and germanium thedeposition rate is increased, and a lower deposition temperature may beused to produce the requisite clear fused deposit.

The manufacture of doped silica by a method involving hydrogen-freedeposition upon the bore of a silica substrate tube produces fibre withspectral characteristics which show a general fall in attenuation withincreasing wavelength in the region from 0.7 to 1.3 microns. This fallis attributed to the effects of Rayleigh scattering. Superimposed onthis general fall are a number of absorption peaks attributed toresidual amounts of hydroxyl contamination. Such absorption peaks occurat 0.95, 1.25, and 1.4 microns. In the region beyond the 1.4 micron peakthere is a further window which may extend to about 1.8 microns. Beyondthis the attenuation begins to rise again due to different absorptionphenomena associated with silica and its deliberately introduceddopants. The fundamental vibrational absorption peak of the Si--O bondis at 9.0 microns, but its absorption tail extends into the 1.0 to 2.0micron region. The B--O, P--O, and Ge--O bonds show similar absorptionpeaks at respectively 7.3, 8.0 and 11.0 microns. It is found that, as aresult of this these different absorption peaks associated with thesebonds, the extent and depth of the windows in the region immediatelybeyond the 1.4 micron hydroxyl group absorption peak depends heavilyupon the composition of the material in which the optical energypropagates.

FIG. 1 depicts the spectral characteristic of a multi-mode graded indexfibre having a core of silica doped mainly with germania but alsoincluding about 1 mole % oxide of phosphorus. The cladding is of silicadoped with oxide of boron. FIG. 2 depicts the spectral characteristic ofa single mode fibre having a germania doped silica core, and a claddingof silica doped with oxides of boron and phosphorus to provide an indexmatching that of undoped silica. A comparison of these twocharacteristics shows that the window beyond 1.4 microns of the FIG. 1fibre is comparatively wide and deep because the optical energypropagates almost exclusively in the material of the core, whereas thecorresponding window of the FIG. 2 fibre is shallower and narrowerbecause a significant proportion of the optical energy propagates in thecladding where it suffers from the absorption tails of the B--O and P--Obonds. This attribute of doping with oxides of phosphorus and boron,which is seen to be a disadvantage in the fibre of FIG. 2, may howeverbe used to advantage in the provision of the absorption layer of fibresconstructed according to the present invention.

According to one preferred example of the invention, a 14 by 12 mmdiameter silica tube 30 (FIG. 3) approximately one meter long iscarefully cleaned, dried, and mounted in a special lathe havingsynchronously driven head- and tail-stocks. Oxygen is passed down thetube which is rotated while an oxy-hydrogen flame is slowly traversedalong its length. This ensures that the tube has clean dryflame-polished bore ready to receive a layer 31 of glass which will formthe optical absorption layer in the completed fibre.

This layer 31 is formed of silica doped by co-deposition with oxides ofphosphorus and boron. Oxide of boron is included at a concentration ofabout 10 mole % because its absorption tail reaches further into thespectral region of interest than that of the oxide of phosphorus. Oxideof phosphorus is included at a concentration typically lying in therange 10 to 15 mole % to provide a further contribution to the requisiteabsorption and to raise the refractive index of the deposit above thatof undoped silica. In determining the proportions of dopants to beemployed it is necessary to take account of the way they affect theproperties of the deposit. Both these dopants reduce the viscosity ofthe deposit and increase its expansion coefficient. If too pronounced,the former effect can give rise to problems when later collapsing thebore of the coated tube in a manner to retain circular symmetry, whilethe latter effect can give rise to problems of stress cracking. Thereagents for forming layer 31 are oxygen gas and silicon tetrachloride,boron tri-bromide, and phosphorus oxychloride vapours. The vapours aretransported in oxygen as a carrier gas by bubbling separate streams ofoxygen through the three liquids. These are then mixed with a furtherstream of oxygen which acts as a diluent. The reaction between thevapours and oxygen does not occur spontaneously at room temperature, butis promoted by the action of the heat of the oxy-hydrogen flame whichcontinues to be traversed along the tube. In this way a number oftraverses are used to build up a layer of adequate thickness. Thetemperature of the flame and the rate of its traversal are carefullycontrolled to provide just enough heat to ensure that the deposit isformed as a clear coherent glassy layer.

Next the composition and proportion of the reagents are changed in orderto deposit a layer 32 which is to form the material of the opticalcladding of the completed fibre. This layer 32 is a layer of silicadoped primarily with germania. Typically it contains about 1 to 2 mole %germania, and generally it includes not more than about 1 mole % oxideof phosphorus, but no oxide of boron. The small amount of oxide ofphosphorus, typically about 0.2 mole %, is retained because at thisconcentration its absorption is not a serious disadvantage, whereas itspresence is useful in lowering the temperature at which the layer can bedeposited in a clear glassy form. Too high a deposition temperature isto be deprecated because this leads to problems of distortion of thesubstrate tube during the deposition process.

After deposition of the layer 32, which proceeds in the same way as thedeposition of layer 31, the composition of the reagents is again changedfor the deposition of layer 33. This is the layer whose material is toform the optical core of the completed fibre, and contains a higherproportion of germania doping in order to provide the requisite increasein refractive index over that of layer 32. Typically the composition ofthe core layer 33 has about 4 mole % more germania than is present inthe cladding layer 32, but the same concentration of oxide ofphosphorus.

Next the bore of the coated tube is collapsed to form a solidcross-section optical fibre preform. This is brought about using ahigher temperature flame to soften the wall of the tube so that itcollapses under the effects of surface tension. Several traverses of theflame are employed to bring about complete collapse of the bore, andduring the initial stages of this collapse it is preferred to maintain aflow of germanium tetrachloride and oxygen through the tube partly toreplenish dopant lost by volatilisation and partly to provide a slightexcess pressure within the bore in order to ensure circular symmetry ismaintained during the collapse process. The manner in which thiscollapse is preferably performed is described in greater detail in U.S.Pat. No. 4,165,224 (J. Irven - A. P. Harrison 12-2).

The resulting preform is suitable for storage until such time as fibreis required. It is then mounted vertically in a pulling tower andlowered through a furnace at a controlled rate while fibre is drawn fromits heat softened lower end. Preferably the resulting fibre is directlypassed through a coating bath to provide it with a plastics protectivecoating that protects its freshly drawn surface from atmospheric attackand from direct contact with anything else which is liable to damage thesurface.

In a typical example the thickness of the layers deposited upon the tube30 are chosen to provide an optical core diameter of 6.5 microns, anoptical cladding diameter of 45 microns, an absorption layer diameter of65 microns, and an overall diameter, exclusive of protective coating, of125 microns.

I claim:
 1. A method of making an optical fibre preform wherein anoptical absorption glass layer of doped silica that includes oxides ofboron and phosphorus and has an index greater than that of silica isdeposited by vapour reaction upon the bore of a silica substrate tube,wherein a transparent optical cladding glass layer of doped silica thatincludes germania and has a refractive index less than, equal to, or notsubstantially greater than that of the absorption layer is deposited byvapour reaction upon the absorption layer, wherein a transparent opticalcore glass layer of doped silica that includes germania and has arefractive index greater than that of the cladding layer is deposited byvapour reaction upon the cladding layer, wherein each of said vapourreactions is a reaction from which hydrogen and its compounds areexcluded, and wherein the bore of the thus coated tube is collapsed toform a solid cross-section optical fibre preform, and the relativeamounts compositions of the core and cladding glasses deposited are suchthat single mode optical fibre is capable of being drawn from thepreform that is capable of single mode operation at selected wavelengthsin the free-space wavelength range of 1.5 to 1.7 microns in whichsubstantially all of the optical power associated with the evanescentfield of said single mode propagates in said cladding glass.
 2. Anoptical fibre preform made by the method claimed in claim 1 wherein theabsorption layer consists of silica doped essentially exclusively withoxides of boron and phosphorus.
 3. An optical fibre preform made by themethod claimed in claim
 1. 4. An optical fibre drawn from a preform asclaimed in claim 3.